Ferromagnetic materials

ABSTRACT

A ferromagnetic material having the formula MGa 2-x  As x  where 0.15≦x≦0.99 and M represents one of Fe 3 , Fe 3  partially substituted by manganese or Fe 3  partially substituted by cobalt.

This invention relates to ferromagnetic materials.

Ferromagnetic materials display a marked increase in magnetisation in anindependently established magnetic field. Ferromagnetic materials may beused in a wide variety of uses including motors or galvanometers. Thetemperature at which ferromagnetism changes to paramagnetism is definedas the Curie Temperature, T_(c).

Ferromagnetic materials based on rare earth elements may have CurieTemperatures up to 700°-800° C., but they oxidise [Goldschmidt ReportReviews Information 4/75 no. 35 and 2/79 no. 48]. The inclusion of ironwithin an alloy is a well established possible method of producing aferromagnetic material. Nd₂ Fe₁₄ B has one of the highest reported CurieTemperatures (315° C.) of rare earth-iron based alloys. Iron may in turnbe used to dope GaAs in order to produce a material with ferromagneticproperties. One of the most recent reports of such material is that ofI. R. Harris et al. in the Journal of Crystal Growth 82 pp 450-458 1987.This publication reported the growth of Fe₃ GaAs as a ferromagneticmaterial (Curie Temperature=about 100° C.) and discussed this alloy withreference to previous work carried out on iron doped GaAs.

The present invention provides an improved stable ferromagnetic GaAsbased material with an increased Curie Temperature.

According to this invention a ferromagnetic material comprises Ga and Asand a balance apart from impurities of M, having a formula M₃ Ga_(2-x)As_(x) where x has the range 0.15≦x≦0.99 and where M represents iron ora component of the ferromagnetic material where iron is partiallysubstituted by manganese.

Where M₃ represents Fe₃ and x is a value within the continuous range0.15≦x≦0.99, then x would have the preferred range of 0.15≦x≦0.85. Themost preferential range for x in this alloy may be expressed as0.15≦x≦0.75.

Where M₃ represents Fe₃ and the range of x is 0.21≦x≦0.99, as castmaterial consists of single phase Fe₃ GaAs with an eutectic mixture atthe grain boundaries. In the range 0.15≦x≦0.21 for the same alloy the ascast material exhibits phases in addition to an eutectic mixture atgrain boundaries.

In as cast material where M₃ represents Fe₃ and the range of x is0.85≦x≦0.99, the predominant phase is hexagonal B8₂ -type Fe₃ Ga_(2-x)As_(x) with a minimal amount of the phase GaAs. Within the B8₂ -type(Ni₂ In-type) the In-type sub-lattice is filled by a combination of Gaand As atoms and three quarters of the two nickel type sites are takenup by the iron atoms.

Lattice structural transition (ordering) occurs within the compositionrange of 0.75≦x≦0.85. The structure is still hexagonal, but there is achange of the a and c spacings such that a₂ =2a₁ and c₂ =c₁, where a₁and c₁ are the a and c spacings of the B8₂ -type structure and a₂ and c₂are the a and c spacings of the new structure. In the composition range0.15≦x≦0.75 the ordering process is complete.

The ferromagnetic material Fe₃ Ga_(2-x) As_(x) may subsequently bevariously heat treated in order to achieve higher Curie Temperatures.Suitable annealing temperatures would be between approximately 600° C.and 900° C. Where M₃ represents partial substitution of iron withmanganese, then this substitution is used to maintain high CurieTemperatures.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention will now be described by way of example only withreference to the accompanying diagrams of which:

FIG. 1 is a schematic representation of Liquid Encapsulation Czochralski(LEC) growing equipment.

FIG. 2 is a graph of the saturation magnetisation of M₃ Ga_(2-x) As_(x)against the atomic percentage of Gallium for as cast material where M₃represents Fe₃.

FIG. 3 is a graph of the variation in Curie Temperature with increasingGallium content for as cast material where M₃ represents Fe₃.

FIG. 4 is a graph of the a-spacing versus the atomic percentage ofGallium in the alloy for as cast material where M₃ represents Fe₃.

The ferromagnetic material M₃ Ga_(2-x) As_(x) may be produced usingtypical methods such as casting or single crystal growth. Both methodsrequire encapsulation of melt constituents to prevent loss of arsenicfrom the melt whilst in a furnace environment. Boric oxide is an exampleof a commonly used encapsulation material.

The Liquid Encapsulation Czochralski technique for growth of singlecrystal material may be used for the growth of the alloy M₃ Ga_(2-x)As_(x), and has been described in U.K. Patent Number 1 113 069. As shownin FIG. 1, the melt constituents 1 (Fe, Ga and GaAs) of applicableratios are placed in a silica crucible 2 and covered with boric oxide 3.The crucible 2 and contents 1 are then heated by electric heaters 4 fedthrough a power supply 5. An orientated seed 6 is lowered into thepressurised chamber 7 by a motor 8. When the seed 6 has been partiallyimmersed in the molten alloy 1, controlled growth takes place byrotating and retracting the seed 6 away from the melt 1, through theencapsulant 3 and into the pressurised chamber environment 7. Thisresults in a single crystal, or near single crystal, boule 9. All growthprocedures are controlled by a control panel 10.

Specific compositions will now be given by way of example only where allexamples are as cast material except Example 6

EXAMPLE 1 Fe₃ Ga₁.85 As₀.15

This composition has a saturation magnetisation of 84 emu g⁻¹ at 298 K.(FIG. 2) and a Curie Temperature of 431° C. (FIG. 3).

EXAMPLE 2 Fe₃ Ga₁.79 As₀.21

This composition has a saturation magnetisation of 97 emu g⁻¹ at 298 K.(FIG. 2), a Curie Temperature of 370° C. (FIG. 3) and an a-spacing of4.07A (FIG. 4).

EXAMPLE 3 Fe₃ Ga₁.5 As₀.5

This composition has a saturation magnetisation of 88 emu g⁻¹ at 298 K.(FIG. 2), a Curie Temperature of 240° C. (FIG. 3) and an a-spacing of4.055A (FIG. 4).

EXAMPLE 4 Fe₃ Ga₁.35 As₀.75

This composition has a saturation magnetisation of 72 emu g⁻¹ at 298 K.(FIG. 2), a Curie Temperature of 232° C. (FIG. 3) and an a-spacing of4.048A (FIG. 4).

EXAMPLE 5 Fe₃ Ga₁.1 As₀.9

This composition has a saturation magnetisation of 79 emu g⁻¹ at 298 K.(FIG. 2), a Curie Temperature of 215° (FIG. 3) and an a-spacing of4.033A.

EXAMPLE 6 Fe₃ Ga₁.4 As₀.6

Alloys may be variously heat treated to homogenise the microstructure.The heat treatment may occur within a vacuum or without a vacuum. Theheat treatment may require an air, inert gas or arsenic ambient at airor other pressures, or a flowing medium of any of these. The annealingtemperatures employed is dependent upon the annealing environment usedand the material properties required.

This composition in the as cast state has a Curie Temperature of 244° C.After annealing the example at about 600° C. in a vacuum of 10⁻⁶ Torrfor three days the Curie Temperature increases to 282° C.

EXAMPLE 7 Fe₂.7 Mn₀.3 Ga₁.85 As₀.15

This composition has a saturation magnetisation of 94 emu g⁻¹ at 298 K.and a Curie Temperature of 416° C.

EXAMPLE 8 Fe₂.7 Co₀.3 Ga₁.85 As₀.15

This composition has a saturation magnetisation of 71 emu g⁻¹ at 298 K.and a Curie Temperature of 346° C.

We claim:
 1. A ferromagnetic material having the formula MG_(2-x) Ga_(x)comprising Ga and As and the balance, apart from impurities, of M; wherex has the range of 0.15≦x≦0.85 and where M represents Fe₃.
 2. Theferromagnetic material according to claim 1 where x has the range0.15≦x≦0.75.
 3. The ferromagnetic material according to claim 1 or claim10 where the ferromagnetic material has been annealed in a temperaturerange of about 600° C. to 900° C.
 4. The ferromagnetic materialaccording to claim 3 where the ferromagnetic material was annealed in avacuum.
 5. The ferromagnetic material according to claim 3 where theferromagnetic material was annealed in an ambient atmosphere selectedfrom air, arsenic and inert gas.
 6. The ferromagnetic material accordingto claim 5 where the ambient atmosphere was a flowing medium.
 7. Theferromagnetic material according to claim 3 where the ferromagneticmaterial was annealed in a vacuum of 10⁻⁶ Torr for three days at atemperature of about 600° C.
 8. A ferromagnetic material having theformula MGa_(2-x) As_(x) comprising Ga and As and the balance, apartfrom impurities, of M; where x has the range 0.15≦x≦0.99 and where M isFe₃ partially substituted by manganese or Fe₃ partially substituted bycobalt.
 9. The ferromagnetic material according to claim 8 where x has arange 0.15≦x≦0.85.
 10. The ferromagnetic material according to claim 8where x has a range 0.15≦x≦0.75.
 11. The ferromagnetic materialaccording to claim 8 where the ferromagnetic material has been annealedin a temperature range of about 600° C. to 900° C.
 12. The ferromagneticmaterial according to claim 11 where the ferromagnetic material wasannealed in a vacuum.
 13. The ferromagnetic material according to claim11 where the ferromagnetic material was annealed in an ambientatmosphere selected from air, arsenic and inert gas.
 14. Theferromagnetic material according to claim 13 where the ambientatmosphere was a flowing medium.
 15. The ferromagnetic materialaccording to claim 11 where the ferromagnetic material was annealed in avacuum of 10⁻⁶ Torr for three days at a temperature of about 600° C.